Michael Yochpaz

## Type of changes - [ ] Bug fix - [ ] New feature - [x] Documentation / docstrings - [ ] Tests - [ ] Other ## Checklist -...

# Summary This PR adds a `default_user_agent` function (corresponding to [requests.utils.default_user_agent](https://github.com/psf/requests/blob/main/src/requests/utils.py#L891-L897)) and allows it to be imported using `from httpx import default_user_agent` to allow different projects to check what user-agent...

[FEAT]: Add Support to Anthropic & OpenAI Batch APIs

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### What would you like to see? Hey there! First off, thank you for working on this great project :) Is it possible to add support for Batch APIs, provided...